Polymer materials into which inorganic fillers are incorporated appear in many technical and other uses. The inorganic fillers here often not only have a substitution and dilution function, but serve to modify the polymeric matrix material in many respects or impart to its certain properties for the first time. A frequent and well-known example of this is the pigmenting of polymer materials and the objects produced therefrom with inorganic pigments for coloring and/or imparting certain optical or visually esthetic properties.
Modification of polymer materials, in particular of those based on polyester, intended for production of films is in the foreground of the present invention. Films of this type have diverse uses, for example as carrier films for magnetic recording media, such as audio, video and computer tapes and floppy disks and the like, as a dielectric in capacitors, as carrier films for reprographic purposes, such as, for example, thermal printing tapes, as embossing and release films, and last but not least as packaging material.
Films having a uniform and completely smooth surface in general have the property of adhering to themselves and to objects with which they come into contact. This results in poor slip properties, which manifest themselves adversely during winding and unwinding operations even during production and also during further processing and later use. It is generally known prior art for films and surfaces thereof to be modified by incorporation of inorganic particles such that they have a certain surface roughness appropriate for their requirements. Particles can be incorporated in appropriate dimensions and amounts at the stage of the film raw material or during film production. It is essential that in the end a sufficient amount of particles are located on or in the film surface such that they project above the plane of the surface to a certain extent and thus act as it were as spacers. In the case of magnetic recording materials, uniform slip over magnetic heads and other tape-guiding components is thus ensured. EP 257611 and EP 261430 describe, for example, such a modification of polyester films for such intended uses with SiO.sub.2 particles.
However, films of the type described above have serious disadvantages. Because of only inadequate adhesion of the particles in the film surface, these are abraded away when the film is used. As a result, on the one hand the slip properties of the film deteriorate during use. On the other hand, the abrasion leads to disturbances, which is serious precisely in the case of magnetic recording media, since mechanical, electromagnetic and electronic components which come into contact with the magnetic tape can thereby be contaminated, damaged or destroyed. Thirdly, precisely in the case of very thin films, the depressions in which the particles are located or which they leave behind in the film surface after abrasion are areas of mechanical weakening or other forms of disturbance. These can lead to cracks and, in the case of magnetic tapes, additionally to disturbances in the magnetic properties going as far as a loss of information, and, in the case of capacitor films, to disturbances in the dielectric properties going as far as electrical breakdown.
Another problem is the following:
During production, the films are often subjected to mono- or biaxial stretching processes to improve their tensile and tear strength. This applies in particular to films of polyester, in particular of polyethylene terephthalate, which, with mono- or biaxial orientation, are preferred in the abovementioned uses because of superior properties with respect to tensile strength, tear strength, elasticity modulus, transparency and chemical and thermal resistance.
During stretching, the areas where the inorganic particles are located are drawn out to cavity- or trough-like depressions, in the centers of which particles with correspondingly reduced adhesion are localized. This of course leads to an increased detachment of the particles and, as a consequence, to an intensification of the abovementioned adverse effects. SEM photographs of stretched films show very clearly the drawn-out depressions with the particles and also the empty depressions, so-called "voids".